Proving A Hydraulics Equation

Prove that the resultant force F on a plane surface of area A immersed vertically in a liquid of density (rho) is given by the equation:
F=rho.g.A.z
Where 'z' is the depth to the centroid of the plane area below the liquid surface level.

I under stand how F is found using this equation and how the force changes depending on the z value. But I don't know how to put this answer in a suitable way for my assignment.